The present invention relates to measuring air mass flow rate and, more particularly, to a mass air flow sensor which maximizes the thermal impedance of the conductive path existing at the ends of flow sensor resistors and a method of manufacturing the same.
Automotive engine control systems frequently make use of a mass air flow sensor to sense the mass flow rate of air entering the intake duct to the engine. One type of mass air flow sensor employs two temperature sensitive resistors. The first resistor, referred to as the ambient sensor resistor, is essentially unheated by the small monitoring current it carries, so its resistance is a function of the temperature of the air flow in which it is immersed. The second resistor, referred to as the heated resistor, is also immersed in the air flow, but carries an appreciable heating current. When a suitable control circuit is employed to keep the temperature difference between these two resistors constant by varying the current through the heated resistor, this heating current is the measure of the mass flow rate of the air carrying heat away from the heated resistor. If the sensor is mounted in a selected location in a duct where flow conditions are repeatable, a calibration curve can be obtained relating flow through the duct to heating current. Subsequently, monitoring heating current permits determination of mass air flow through the duct.
One known air velocity sensor is a hot film anemometer-type mass air flow sensor. In such hot film wire or hot film anemometer, a thin film wire sensor is deposited on a substrate such as quartz or glass. It is common to use a very fine platinum or tungsten wire freely supported or wound on a ceramic bobbin and maintained at a certain temperature above the intake air temperature by electronic sensing and feedback circuits. Any change in the airflow alters the cooling effect of the air on the heated wire. An electronic circuit can sense the change in the heat transfer rate by monitoring changes in the heating current to maintain the temperature of the wire at a set value.
Hot film anemometer sensors have generally had a low speed of response as a result of the low thermal conductivity of the quartz or glass fiber. Further, great care must be taken in handling the thin film coated quartz fiber in manufacturing the sensor elements. Connecting the quartz fiber or fine wire to a supporting structure and making electrical contact involves time consuming and delicate operation. Electronics used to amplify and power the sensor are placed external to the hot film anemometer sensor, and interconnection between the electronics and the sensor is made by wires. The connections between the sensor element and the interconnection wires are a source of loss of reliability and increased unit costs. This results in limited manufacturing production capacity and increased unit costs.
Other air velocity sensors that are commercially available are commonly of the single hot wire or thermistor type and are typically mounted on the end of a long probe for insertion into an air stream. The temperature drop and the associated change in electrical resistance caused by the cooling effect of the air stream is a measure of the airflow velocity. In these devices the elements are fully exposed to the air stream and are susceptible to breakage and contamination; also the temperature change with airflow is quite nonlinear, and the resulting electrical signal must be carefully linearized by an integrated circuit. Furthermore, such devices are quite expensive and not suitable for large scale mass production.
As a result of these problems, many attempts in the art have been made to produce a flow sensor which utilizes silicon and its semiconductor properties, or a pyroelectric material. These attempts improved the state of the art in some respects, yet remained deficient with respect to many of the characteristics desired in a modern flow sensor. As is known in the semiconductor industry, very fine platinum or tungsten wires may be freely supported on a silicon chip for use in sensors. Any change in the air flow alters the cooling effect of the air on a heated wire. An electronic circuit can sense this change in heat transfer rate and change the heating current so as to maintain the temperature of the heated wire in a prescribed relationship to the temperature of an ambient sensing wire, to permit measurement of mass air flow.
This particular application has been extremely useful in the automotive industry for measuring engine air flow. In that regard, U.S. Pat. No. 4,594,889, issued to McCarthy, provides a method of fabricating a mass air flow sensor including the step of forming a generally planar silicon substrate. A pair of spaced openings are formed through the silicon substrate so that a relatively elongated, thin wire-like silicon region remains between the openings. After a silicon dioxide coating is formed on this silicon substrate, an elongated metal coating is applied to the silicon dioxide on the wire-like silicon region. The device fabricated thereby may be used as a low cost, high speed sensing element, such as a mass air flow sensor for measuring air flow in an electronic engine control system. However, the McCarthy flow sensor incorporates an elongated, thin silicon member which is susceptible to breakage upon impact of particles moving with the fluid to be measured, such as dust particles travelling with the air flow in the intake of an automobile engine control system. Also the heat loss conducted out of the ends of the heated resistor is appreciable due to the high thermal conductivity of silicon.
Another attempt at producing a suitable flow sensor is disclosed in U.S. Pat. No. 4,498,337, issued to Gruner. The Gruner reference discloses a flow sensor having a flat, metal support base or carrier, such as titanium, which supports thin film resistors or sensing elements. Interposed between the metal support base and resistors is an electrically insulating layer such as glass. So as to minimize conductive heat loss through the support base to the structure surrounding the center, portions of the sensor edges which are in contact with the surrounding structure are removed. Although Gruner's use of a metal support base may provide the flow sensor with some degree of mechanical strength, the Gruner flow sensor does not appear to have achieved the combination of high mechanical strength and low conductive heat loss as is desired. More specifically, while Gruner uses a continuous flat sheet to support the electrical insulating layer and sensing/heating elements, there remains a need for not only an electrical insulating layer, but also a thermal insulating layer so as to preclude any conductive heat loss from the sensing/heating elements.
It is seen then that there is a need for a flow sensor which maximizes thermal impedance between sensing element and support. Also, there is a need for such a sensor wherein mass production techniques permit economical fabrication of a large number of sensors at once while eliminating several previously necessary steps. Finally, there remains a need in the art for a method of making the aforementioned flow sensor that permits a more rugged sensor while maintaining low heat conduction to the support.